1. Field of the Invention
This invention relates to earth boring or rock bits--in particular to improvements in those bits having extended nozzles used to discharge drilling fluid closely against the bottom of a borehole.
2. Description of the Prior Art
The earliest rotary rock bits discharged drilling fluid in the borehole to cool the bit and wash cuttings to the surface of the earth. After World War II and the advent of high pressure pumps, the so-called "jet" bit improved drilling rates and bit life by discharging high velocity streams of fluid directly against the bottom of the bore hole.
It has been shown that further improvements are achieved by locating the nozzles close to the bottom of the borehole. As the nozzle is placed closer to bottom the pressure under the impinging jet increases. Best results are obtained when the jet is two to six nozzle diameters off bottom. For this reason the extended jet nozzle was developed to place the nozzles at the lower end of a tube that extends into proximity with the bottom of the hole.
On conventional three cone bits, which are designed to provide maximum bearing capacity and cutting structure intermesh, the space left for the placement of the nozzle extensions is a narrow curved passage.
Existing designs for nozzle extensions which fit into this space have disadvantages that should be overcome: (1) The fluid velocity in the curved sections of the extensions is frequently above the threshold level for erosion. (2) The fluid changes direction more than once through small radius curves, which intensifies erosion. (3) The section modulus of the nozzle extensions is essentially constant over the full length of the tube, resulting in low strength at the base of the tube, which can be regarded as a cantiliver beam when exposed to concentrated loads at its lower end due to contact with the borehole wall or debris on the bottom of the borehole. As a consequence extended nozzle bits have a reputation of short life and low reliability due to premature erosion as well as mechanical damage and breakage of the nozzle extension tubes. Frequently, the gain in penetration rate cannot offset the loss in bit life and increased risk.